1. Field of the Invention
The invention relates to a pour tube for use in the continuous casting of molten metal. More particularly, the invention describes an article and method for improving flow characteristics of the molten metal.
2. Description of the Prior Art
In the continuous casting of metal, particularly steel, a stream of molten metal is typically transferred via a refractory pour tube from a first metallurgical vessel into a second metallurgical vessel or mold. Such tubes are commonly referred to as shrouds or nozzles, and possess a bore through which the metal passes. One important function of a pour tube is to discharge the molten metal in a smooth and steady manner without interruption or disruption. A smooth, steady discharge facilitates processing and can improve the finished product.
Factors, which can disrupt the steady discharge, include asymmetric flow of molten metal and clogging of the bore. Asymmetric flow may develop before or after the stream is in the bore. For example, while flowing through a bore, a stream may develop higher fluid velocity near the centerline of the bore than along the sides of the bore, or lower velocity on one side of the centerline as compared to the opposite side, or higher fluid velocity off the centerline. The disparate velocities can cause pulsing and excessive turbulence upon exiting the bore, thereby complicating processing and decreasing the quality of the finished product. Throttling devices, such as stopper rods or slide gate valves, can partially obstruct the entrance to the bore, and cause the steam of molten metal to enter the bore off the centerline. The stream can flow preferentially down one side of the bore, and exit asymmetrically from the pour tube causing surging and turbulence in a mold.
Precipitates may also clog or restrict the bore so as to disrupt steady discharge of the molten metal. In molten steel, precipitates are primarily alumina and other high melting point impurities. Alumina deposits can lead to restrictions and clogging that can stop or substantially impede the smooth and steady flow of molten steel. Tubes may be unclogged using an oxygen lance; however, lancing disrupts the casting process, reduces refractory life, and decreases casting efficiency and the quality of the steel produced. A total blockage of the bore by precipitates decreases the expected life of the pour tube and is very costly and time-consuming to steel producers.
Prior art attempts to improve flow include both chemical and mechanical means. For example, flow may be improved by reducing alumina precipitation and subsequent clogging. Prior art has injected inert gas into the pour tube to shield the flow from the pour tube, thereby reducing precipitation and clogging. Unfortunately, gas injection requires large volumes of gas, complicated refractory designs, and is not always an effective solution. Gas may also dissolve or become entrapped within the metal causing problems in metal quality including pinhole defects in the steel. Alternatively or in combination with gas injection, prior art has lined the bore with refractory compositions that are claimed to resist alumina buildup. Compositions include lower melting point refractories, such as CaOxe2x80x94MgOxe2x80x94Al2O3 eutectics, MgO, calcium zirconate and calcium silicide, that slough off as alumina deposits on the surface. These compositions tend to crack at high temperature, and, during casting, they may hydrate and dissipate. For these reasons, their useful life is limited. Other surface compositions that claim to inhibit alumina deposition, include refractories containing SiAlON-graphite, metal diborides, boron nitrides, aluminum nitride, and carbon-free compositions. Such refractories can be expensive, impractical, and manufacturing can be both hazardous and time consuming.
Mechanical designs for improving flow include U.S. Pat. No. 5,785,880 to Heaslip et al., which teaches a pour tube having a diffusing geometry that smoothly delivers a stream of molten metal to a mold. Alternative designs include EP 0 765 702 B1, which describes a perforated obstacle inside the bore that deflects the stream from a preferred trajectory. Both references attempt to control the introduction of molten metal into a mold by mechanically manipulating the stream of molten metal. Neither describes alumina clogging or the reduction of alumina clogging.
Prior art also includes designs that claim to improve flow by reducing alumina deposition in the bore. These designs include pour tubes with both conical and xe2x80x9csteppedxe2x80x9d bores. U.S. Pat. No. 4,566,614 to Frykendahl teaches an inert gas-injection nozzle having a conical bore intended to reduce xe2x80x9cpulsationsxe2x80x9d in the gas flow. Smoother gas flow into the bore is said to reduce clogging. xe2x80x9cSteppedxe2x80x9d designs include pour tubes that have discontinuous changes in bore diameter. Stepped designs also include pour tubes having a spiral bore. JP Kokai 61-72361 is illustrative of stepped pour tubes, and describes a pour tube having a bore with at least one convex or concave section that generates turbulent flow in the molten metal. Turbulent flow, as contrasted with lamina flow, is described as reducing alumina clogging. U.S. Pat. No. 5,328,064 to Nanbo et al. teaches a bore having a plurality of concave sections separated by steps having a constant diameter, d. Each section has a diameter greater than d, and preferably the diameters of the sections decrease along the direction of flow. The steps are described as generating turbulence that reduces alumina clogging.
Prior art stepped designs show turbulent flow only at a step or the beginning or end of a section. None describe turbulent flow away from these features, including at the middle of the section. Non-turbulent flow permits alumina to buildup on the surface of the bore, and can lead to clogging of the bore away from the step. Further, no prior art design simultaneously describes a pour tube that reduces asymmetric flow of molten metal passing through the pour tube""s bore and the relationship between reduced asymmetric flow and alumina clogging.
A need persists for a refractory pour tube that inhibits alumina deposition along the entire length of the bore. Ideally, such a tube would also improve the flow of molten metal into a casting mold.
The present invention relates to an article and method for improving flow of a stream of molten metal and reducing alumina precipitation in a bore of the article. In a broad aspect, the article comprises a pour tube having a bore comprised of a series of fluidly connected sections each of which converges and diverges to continuously alter and diffuse the contained stream.
In one aspect, the pour tube has a bore comprised of a series of fluidly connected sections where each section has a sharply converging portion and a slowly diverging portion. The combination of the converging and diverging elements can reduce flow asymmetry, reduce alumina deposition in the bore, and inhibit surging and asymmetry in the flow exiting the bore. In one embodiment, the converging portion is upstream of the diverging portion.
The converging portion comprises a step inclined at a sharp angle from the center axis. The diverging portion comprises a length and an inside surface that, in the direction of flow, diverges from the center axis at a diverging angle which is significantly smaller than the sharp angle of the inclined step. The diverging angle is large enough to diffuse the stream of metal, but small enough to prevent pressure drops or separation of the stream. Each section has inlet and outlet cross-sectional areas. From section to section, the inlet and outlet areas may increase, decrease, or remain relatively constant in the direction of flow, thereby reducing, increasing, or maintaining the mean velocity of the contained stream as desired for the flow exiting the bore.
In another aspect of the invention, the pour tube has a bore comprised of a series of fluidly connected sections, where each section has a sharply converging means and a slowly diverging means. The converging means deflects the stream toward the center axis of the bore, and the diverging means directs the stream away from the center axis without separation of the stream. In a further embodiment of the invention, he pour tube has a bore comprising a series of fluidly connected, frusto-conical sections with a converging means between each section.
The method of the invention has a pour tube with a bore comprised of a series of fluidly connected sections, and includes converging the stream of molten metal at the inlet of each section and diverging the stream without separation along the length of the section.
Other details, objects and advantages of the invention will become apparent as the following description of a present preferred method of practicing the invention proceeds.